Close the lease-client gaps the port reviews found

[bpapillon]

Python leg of SCH-7530, matching schematic-node #194 commit for commit.

• Re-read the lease after winning the extend flight, so a stale row cannot send a second extend.
• Write the reservation hash and its TTL in one MULTI; the index writes stay outside.
• Feed a preflight set on the evaluation context to the local engine through the same merge as REST.
• Bound the drain and the local lease release by one shutdown budget.
• Round a fractional preflight quantity up at the WASM boundary; the engine rejects floats there.
• Round the hold and the settle debit up to whole event units, so the local ledger moves by exactly what the track event bills. SPEC.md and the vectors are copied from Node, with the new fractional-usage.json.
• Resolve company keys through the cache first and only fall back to a comp_-prefixed value as the Schematic id, the order the server uses.
• A joiner joins an in-flight extend only when that extend covers its shortfall, and after two joins sends exactly one extend of its own.
• Move the cached company metric only when a settle landed locally.
• Cap a joiner's wait at the caller's own timeout while the shared extend runs on.

Already correct in Python and pinned by the new vector: no wait on a cache miss while disconnected, and the settle quantity rounding up on the wire.